Modified Abstract. This research group recently published the first description of diagonal capillary electrophoresis. Diagonal capillary electrophoresis is a form of two-dimensional capillary electrophoresis that employs identical separation modes in each dimension. The distal end of the first capillary incorporates an enzyme-based microreactor. Analytes that are not modified by the reactor will have identical migration times in the two capillaries and will generate spots that fall on the diagonal in a reconstructed two-dimensional electropherogram. Analytes that undergo enzymatic modification in the reactor will have a different migration time in the second capillary and will generate spots that fall off the diagonal in the electropherogram. Dephosphorylation and desialylation result in the loss of a negative charge, which is trivial to detect using capillary electrophoresis. In this proposal, we will develop diagonal capillary electrophoresis as a robust tool for the determination of the phosphorylation status of complex samples. The technology offers significant advantages compared with alternatives. Most importantly, the technology provides accurate estimates of the extent of modification without artifact due to differences in ionization efficiency of the native and modified peptides. Also, the technology is fully automated: once the sample is loaded into the instrument, no fraction collection, pipetting, or other sample manipulations are performed. Finally, the technology employs capillary electrophoresis, which appears to be better suited than reversed-phase liquid chromatography for the analysis of peptides containing charged modifications.